The current proposal is a five year competitive renewal application by the Massachusetts General Hospital to support its Mallinckrodt General Clinical Research Center. The GCRC has served the needs of clinical investigators at the Massachusetts General Hospital for more than 65 years. It has been funded by the National Institutes of Health for thirty years, first as a metabolic research center (NIAMD) and since 1978 as a General Clinical Research Center (NCRR). The current application requests almost 2000 patient bed days (per diem research unit) and 1300 outpatient visits. The 52 protocols that will utilize these resources represent clinical research from 65 investigators located in 13 units and 10 departments at the MGH. For the current submission, a major effort has been made to expand the number of investigators from departments outside of the historically most active users of the Mallinckrodt GCRC. Therefore, in addition to protocols in endocrinology and metabolism, new clinical research programs have been initiated by the Departments of Orthopedics, Pediatric Surgery, and Psychiatry and by the Infectious Diseases and Oncology Units of the Department of Medicine. The Mallinckrodt GCRC provides facilities for a broad array of clinical research activities, particularly research requiring frequent, timed collections of blood and other body fluids, innovative and complex diagnostic, therapeutic and physiological techniques, careful control of diets, controlled sleep schedules or light environment. Many of the new protocols include patients who are generally more ill than those studied in the past. In addition to its recently renovated facilities, the GCRC offers expert nursing care, sophisticated dietary support, biostatistical support, a new computer system (RS-1), a Core Laboratory to serve the needs of the clinical investigators, and other administrative support necessary to perform clinical investigation. The GCRC is very active in training physicians, nurses, dietitians, and students in clinical investigation. The educational efforts on the GCRC include post-graduate training supported through several mechanisms (CAP awards and a privately funded Mallinckrodt Clinical Research Fellowship), mentor based training, a program for medical students, and a didactic teaching program. The Mallinckrodt GCRC has a long history of excellence in clinical investigation and has been the site of meaningful advances in clinical medicine for more than six decades. It will continue to serve as the major site for inpatient clinical investigation at the MGH.